Exhaust gas recirculation (EGR) systems divert a portion of the exhaust gases back to the intake to cool combustion temperatures and reduce throttling losses, thus improving vehicle emissions and fuel economy. In turbocharged engines, an EGR system may include a cooled low-pressure EGR (LP-EGR) circuit wherein exhaust gases are diverted after the gases pass through the turbine of the turbocharger and injected before the compressor upon passage through an EGR cooler. The amount of LP-EGR routed through the EGR system is measured and adjusted based on engine speed and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits.
One example EGR system is shown by Styles et al. in US 20120023937. Therein, LP-EGR is provided at a fixed EGR percentage rate of fresh airflow over a large area of an engine map, including from mid-load down to minimum engine load, even as engine load changes. At higher engine loads, the EGR percentage rate is varied based on engine operating conditions. In addition, at very low engine loads and/or engine idle conditions, no EGR (0% EGR) may be delivered. Such an approach improves transient control and extends the use of EGR over a wider range of operating conditions.
However the inventors herein have identified a potential issue with such an approach. Specifically, there may be transient maneuvers such as accelerations or throttle tip-ins where the engine may enter a high load operating region from a region of low or 0% EGR. Herein, the engine may enter the high load operating region fast enough that the transport delay of EGR filling the intake system can delay EGR entering the combustion chamber in sufficient concentration to provide the required knock mitigation. As a result, excessive engine knock or pre-ignition can occur. To mitigate the knock or pre-ignition, inefficient use of spark retard or combustion mixture enrichment may be required degrading fuel economy, and offsetting the fuel economy benefits of the prior EGR usage. The abnormal combustion events can also erode drive cycle fuel efficiency and potentially damage the engine.
In one example, some of the above issues can be at least partly addressed by a method for an engine comprising: in response to increasing load while operating with lower EGR, increasing EGR, and fueling the engine with split fuel injection per cycle until EGR is higher than a threshold. In this way, knock and pre-ignition during transient tip-ins can be better mitigated.
As an example, an engine system may be configured with a low pressure EGR (LP-EGR) system configured to recirculate a portion of exhaust gas from an exhaust manifold, downstream of an exhaust turbine, to an intake manifold, upstream of an intake compressor via an EGR valve. The exhaust gas may be cooled upon passage through an EGR cooler before being delivered to the intake. Based on engine operating conditions, such as engine speed-load conditions, a target EGR rate may be determined and various engine actuators including the EGR valve may be adjusted to provide the target EGR rate. If there is a transport delay in the delivery of the desired engine dilution, the engine may become knock-limited. To improve knock control on any such maneuver that transitions the engine into a knock-limited regime where the actual EGR rate is less than the EGR rate desired for proper knock mitigation, while the EGR rate is increased to a threshold rate, the engine may be transiently operated with split fuel injection. In particular, until the EGR rate reaches the target EGR rate, fuel may be delivered at least as a first intake stroke injection and a second compression stroke injection. The first intake stroke injection may be adjusted to be lean and at or around spark timing so as to reduce the propensity for cylinder knock or pre-ignition. The second compression stroke injection may then be adjusted to be rich and after spark timing so as to bring the overall combustion air-fuel ratio at or around stoichiometry. Split fuel injection may be continued until the threshold EGR rate is reached, following which single fuel injection may be resumed. Example maneuvers where split fuel injection may improve knock control include a tip-in to high load and high EGR rate conditions from a low-load condition (such as from a zero EGR condition to a fixed EGR rate relative to airflow condition), a tip-in from high load and low EGR rate conditions to high load and high EGR rate conditions, and any EGR delivery errors and delays where engine operation becomes knock-limited.
In this way, split fuel injection may be temporarily used while LP-EGR is ramped in to reduce abnormal combustion events. By using at least a lean intake stroke injection and a rich compression stroke injection around the time of spark, the likelihood of knock and pre-ignition can be reduced while maintaining combustion air-fuel ratio at stoichiometry. By delaying the need for spark retard or enrichment in addressing knock or pre-ignition, significant improvements in fuel economy are achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.